Method of fabricating a semiconductor device having a lateral double diffused MOSFET transistor with a lightly doped source and CMOS transistor

ABSTRACT

Methods and systems for monolithically fabricating a lateral double-diffused MOSFET (LDMOS) transistor having a source, drain, and a gate on a substrate, with a process flow that is compatible with a CMOS process flow are described. In some implementations, a method of fabricating a semiconductor device is provided that includes forming an LDMOS transistor having a first drain with a first drain-side n+ region, a first source with a first source-side n+ region and a first source-side p+ region, and a first gate between the first drain and the first source on the substrate. The method also includes forming an n-type CMOS transistor having a second drain having a second drain-side n+ region, a second source having a second source-side n+ region, and a second gate between the second drain and the second source. In so doing, the LDMOS transistor can be fabricated through a process that can be seamlessly integrated into a sub-micron CMOS process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/681,690, filed Mar. 2, 2007, now U.S. Pat. No. 7,671,411 which claimsthe benefit of U.S. Provisional Application Ser. No. 60/778,732, filedon Mar. 2, 2006. The disclosure of the prior applications are consideredpart of and are herein incorporated by reference in their entireties inthe disclosure of this application.

TECHNICAL FIELD

The present invention relates to semiconductor devices.

BACKGROUND

Voltage regulators, such as DC to DC converters, are used to providestable voltage sources for electronic systems. Efficient DC to DCconverters are particularly needed for battery management in low powerdevices, such as laptop notebooks and cellular phones. Switching voltageregulators (or simply “switching regulators”) are known to be anefficient type of DC to DC converter. A switching regulator generates anoutput voltage by converting an input DC voltage into a high frequencyvoltage, and filtering the high frequency input voltage to generate theoutput DC voltage. Specifically, the switching regulator includes aswitch for alternately coupling and decoupling an input DC voltagesource, such as a battery, to a load, such as an integrated circuit. Anoutput filter, typically including an inductor and a capacitor, iscoupled between the input voltage source and the load to filter theoutput of the switch and thus provide the output DC voltage. Acontroller, such as a pulse width modulator or a pulse frequencymodulator, controls the switch to maintain a substantially constantoutput DC voltage.

LDMOS transistors are commonly used in switching regulators as a resultof their performance in terms of a tradeoff between their specificon-resistance (R_(dson)) and drain-to-source breakdown voltage (BV_(d)_(—) _(s)). Conventional LDMOS transistors are typically fabricatedhaving optimized device performance characteristics through a complexprocess, such as a Bipolar-CMOS (BiCMOS) process or a Bipolar-CMOS-DMOS(BCD) process, that includes one or more process steps that are notcompatible with sub-micron CMOS processes typically used by foundriesspecializing in production of large volumes of digital CMOS devices(e.g, 0.5 μm DRAM production technologies), as described in greaterdetail below. As a result, conventional LDMOS transistors are,therefore, not typically fabricated at such foundries.

A typical sub-micron CMOS process used by foundries specializing inproduction of large volumes of digital CMOS devices, referred to hereinas sub-micron CMOS process, will now be described. A sub-micron CMOSprocess is generally used to fabricate sub-micron CMOS transistors—i.e.,PMOS transistors and/or NMOS transistors having a channel length that isless than 1 μm. FIG. 1 shows a PMOS transistor 100 and an NMOStransistor 102 fabricated through a sub-micron CMOS process on a p-typesubstrate 104. The PMOS transistor 100 is implemented in a CMOS n-well106. The PMOS transistor 100 includes a source region 108 and a drainregion 110 having p-doped p+ regions 112 and 114, respectively. The PMOStransistor 100 further includes a gate 116 formed of a gate oxide 118and a polysilicon layer 120. The NMOS transistor 102 is implemented in aCMOS p-well 122. The NMOS transistor 102 includes a source region 124and a drain region 126 having n-doped n+ regions 128 and 130,respectively. The NMOS transistor 102 further includes a gate 132 formedof a gate oxide 134 and a polysilicon layer 136.

FIG. 2 illustrates a sub-micron CMOS process 200 that can be used tofabricate large volumes of sub-micron CMOS transistors (such as the CMOStransistors shown in FIG. 1). The process 200 begins with forming asubstrate (step 202). The substrate can be a p-type substrate or ann-type substrate. Referring to FIG. 1, the CMOS transistors arefabricated on a p-type substrate 104. A CMOS n-well 106 for the PMOStransistor and a CMOS p-well 122 for the NMOS transistor are implantedinto the substrate (step 204). The gate oxide 118, 134 of each CMOStransistor is formed, and a CMOS channel adjustment implant to controlthreshold voltages of each CMOS transistor is performed (step 206). Apolysilicon layer 120, 136 is deposited over the gate oxide 118, 134,respectively (step 208). The p+ regions of the PMOS transistor and then+ regions of the NMOS transistor are implanted (step 210). The p+regions 112, 114 and n+ regions 128, 130 are highly doped, and providelow-resistivity ohmic contacts. In a sub-micron CMOS process, formationof an n+ region typically occurs through a three-step process in asingle masking and photolithography step as follows: 1) a lightly dopedn-type impurity region is implanted, 2) an oxide spacer is formed, and3) a heavily doped n+ impurity region is implanted. Formation of a p+region occurs in a similar manner. The formation such n+ and p+ regionsallow transistors to have an improved hot carrier performance.

Foundries specializing in production of large volumes of digital CMOSdevices generally have fixed parameters associated with the foundries'sub-micron CMOS process. These fixed parameters are typically optimizedfor the mass production of digital sub-micron CMOS transistors. Forexample, in process step 206, the CMOS channel adjustment implantgenerally has an associated thermal budget that is typically fixed, andhas parameters optimized for mass production of sub-micron CMOStransistors.

As discussed above, conventional LDMOS transistors typically achieveoptimized device performance through a complex process, such as a BiCMOSprocess or a BCD process, that includes one or more process steps thatare not compatible with a sub-micron CMOS process optimized for the massproduction of digital sub-micron CMOS transistors.

FIG. 3A shows a conventional LDMOS transistor 300 fabricated through aBiCMOS process on a p-type substrate 302. The LDMOS transistor 300includes source region 304 with an n-doped n+ region 306, a p-doped p+region 308, and a p-doped P-body diffusion (P-body) 310. The LDMOStransistor 300 also includes a drain region 312 with an n-doped n+region 314 and an n-type well (HV n-well) 316, and a gate 318, includinga gate oxide 320 and a polysilicon layer 322.

In the BiCMOS process, the gate oxide 320, and gate oxide of any CMOStransistors fabricated in the BiCMOS process, is formed prior toimplantation of the n+ region 306 and the P-body 310. The BiCMOSprocess, therefore, allows the gate 318 to serve as a mask duringimplantation of the n+ region 306 and the P-body 310—i.e., the n+ region306 and the P-body 310 are self aligned with respect to the gate 318.The self aligned lateral double diffusion of the n+ region 306 and theP-body 310 forms the channel of the LDMOS transistor 300.

Such kinds of self aligned double diffusions are not easily integratedinto a sub-micron CMOS process because the subsequent drive-in step (orthermal budget) associated with self aligned double diffusions disruptsthe fixed thermal budget associated with sub-micron CMOS process steps(e.g., process step 206) and requires a redesign of the thermal budgetallocated to the sub-micron CMOS process steps. That is, the selfaligned double diffusions generally includes a drive-in step with a longduration and a high temperature that can cause the characteristics ofsub-micron CMOS transistors (e.g., threshold voltages) to shift.

The lateral doping profile in region (a) of the LDMOS transistor 300controls the tradeoff between the on-resistance R_(dson) and thedrain-to-source breakdown voltage BV_(d) _(—) _(s). The vertical dopingprofile in region (b) determines the drain-to-substrate breakdownvoltage BV_(d) _(—) _(sub) of the LDMOS transistor, and the pinch-offdoping profile in region (c) determines the source-to-substratepunch-through breakdown voltage BV_(s) _(—) _(sub) of the LDMOStransistor. The source-to-substrate punch-through breakdown voltageBV_(s) _(—) _(sub) is an important parameter for an LDMOS transistorwith a floating operation requirement, e.g, an LDMOS transistorimplemented as a high-side control switch in a synchronous buck circuitconfiguration.

FIG. 3B shows a conventional LDMOS transistor 330 fabricated through aBCD process on a p-type substrate 332. The LDMOS transistor 330 includessource region 334 with an n-doped n+ region 336, a p-doped p+ region338, and a p-doped P-body 340. The LDMOS transistor 330 also includes adrain region 342 with an n-doped n+ region 344 and an n-type layer (HVn-Epi) 346, and a gate 348, including a gate oxide 350 and a polysiliconlayer 352. As with the BiCMOS process, in the BCD process, the gateoxide 350, and gate oxide of any CMOS transistors fabricated in the BCDprocess, is formed prior to implantation of the n+ region 336 and theP-body 340.

In the BCD process, an n+buried layer 354 can be grown on the p-typesubstrate 332 to improve the source-to-substrate punch-through breakdowncharacteristics of the LDMOS transistor. Such an approach offers animproved tradeoff between the on-resistance R_(dson) and drain-to-sourcebreakdown voltage BV_(d) _(—) _(s) of the LDMOS transistor as thelateral doping profile of the LDMOS transistor can be optimized withoutconstrain on the vertical doping profiles. However, such a BCD processincludes the growth of the HV n− Epi layer 346, and this step isgenerally not compatible with a sub-micron CMOS process.

Another approach used in a BCD process is to utilize an n− layer 360implanted in the drain region 362 of the LDMOS transistor 364 as shownin FIG. 3C. The n− layer 360, n+ region 366, and P-body 368 are selfaligned with respect to the gate 370—i.e., the n− layer 360, n+ region366, and P-body 368 are implanted after formation of gate oxide 372. Theinclusion of the n− layer 360 provides an additional parameter tofurther optimize the tradeoff between the on-resistance R_(dson) anddrain-to-source breakdown voltage BV_(d) _(—) _(s) of the LDMOStransistor. Similar to the n+ buried layer approach of FIG. 3B, theinclusion of the n− layer 360 at the surface provides a method todecouple vertical and horizontal doping constraints.

SUMMARY

In one aspect, the invention is directed to a method of fabricating atransistor having a source, drain, and a gate on a substrate. The methodincludes implanting, into a surface of the substrate, a high voltagen-doped n-well; forming a gate oxide between a source region and a drainregion of the transistor; covering the gate oxide with a conductivematerial; implanting, into the source region of the transistor, ap-doped p-body; implanting, only into the source region of thetransistor, a n-doped lightly doped source; implanting, into the sourceregion of the transistor, a first n-doped n+ region, the first n-dopedn+ region overlapping a portion of the n-doped lightly doped source;implanting, into the drain region of the transistor, a second n-doped n+region; and implanting, into the source region of the transistor, ap-doped p+ region.

Implementations of the invention may include one or more of thefollowing features. The n-doped lightly doped source may extend beneaththe gate oxide, or extend further laterally than the first n-doped n+region beneath the gate oxide. The method may further include forming anoxide spacer on each side of the gate oxide after implanting the n-dopedlightly doped source but before the first n-doped n+ region and thesecond n-doped n+ region. The oxide spacer may be formed prior toformation of the first n-doped n+ region and the second n-doped n+region. The n-doped lightly doped source may be formed after formationof the gate oxide. The first n-doped n+ region and the second n-doped n+region may be implanted using the same mask. In the source region, asurface area of the n-doped lightly doped source, a surface area of thefirst n-doped n+ region and a surface area of the p-doped p+ region maybe located within a surface area of the p-doped p-body. The p-dopedp-body may be implanted after formation of the gate oxide. The methodmay further include implanting, into the source region of thetransistor, a n-doped lightly doped source including abutting a surfacearea of the n-doped lightly doped source with a surface area of thep-doped p+ region, and the first gate region may abut the second gateregion. The n-doped lightly doped source may be self-aligned to thefirst gate region and may not extend laterally into the second gateregion as measured along the surface of the transistor. The n-dopedlightly doped source and the first n-doped n+ region may be implantedseparately using separate masks. The drain region may be formed withouta lightly doped source, and the second n-doped n+ region may beself-aligned to the gate of the transistor. The method may furtherinclude implanting, into the drain region of the transistor, a n-dopedshallow drain. In the drain region, a surface area of the second n-dopedn+ region may be located entirely within a surface area of the n-dopedshallow drain. The n-doped shallow drain may be self aligned to the gateof the transistor and may be implanted after formation of the gateoxide. The n-doped lightly doped source and the first n-doped n+ regionmay be implanted using the same mask.

In another aspect, the invention is directed to a transistor. Thetransistor includes a p-type substrate; a high voltage n-well formed ina surface area of the p-type substrate; a source including: a p-dopedp-body, a p-doped p+ region within the p-body, a first n-doped n+ regionwithin the p-body and abutting the p+ region, and a n-doped lightlydoped source (N-LDS) region overlapping the first n-doped n+ region, adrain including a second n-doped n+ region; and a gate to control adepletion region between the source and the drain.

Implementations of the invention may include one or more of thefollowing features. The drain may further include an n-doped shallowdrain, the second n-doped n+ region being within the n-doped shallowdrain. The second n+ region may extend deeper than the n-doped shallowdrain. The second n-doped n+ region may be self-aligned to the gate ofthe transistor. The first n+ region may be surrounded by the p-body. Thep-body may be deeper than the p+ region, the first n+ region and theN-LDS region. The gate may include a gate oxide between the source andthe drain, and wherein the N-LDS may extend beneath the gate oxide. TheN-LDS region may extend further laterally beneath the gate oxide thanthe n+ region. The n-doped shallow drain may extend beneath the gateoxide. An outer boundary of the n-doped shallow drain may extend furtherlaterally beneath the gate oxide toward the source than an outerboundary of the second n-doped n+ region. The p-doped body may beself-aligned to the gate of the transistor. The n-doped shallow drainmay be self-aligned to the gate of the transistor. The gate oxide may becovered with a conductive material. An outer boundary of the n-dopedlightly doped source may be aligned with an outer boundary of the firstn-doped n+ region. The n-doped lightly doped source may be adjacent toand abut the p+ region. The n-doped lightly doped source may beimplanted only in the source and not in the drain.

In yet another aspect, the invention is directed to a method offabricating a transistor having a source, drain, and a gate on asubstrate. The method includes implanting, into a surface of thesubstrate, a first impurity region with a first volume and a firstsurface area, the first impurity region being of a first conductivitytype; forming a gate oxide between a source region and a drain region ofthe transistor; covering the gate oxide with a conductive material;implanting, only into the source region of the transistor, a secondimpurity region with a second volume and a second surface area in thefirst surface area, the second impurity region being of an oppositesecond conductivity type relative to the first conductivity type;implanting, into the source region of the transistor, a third impurityregion with a third volume and a third surface area and a fourthimpurity region with a fourth volume and a fourth surface area, in thesecond surface area, the third impurity region being of the firstconductivity type, the fourth impurity region being of an oppositesecond conductivity type; implanting, into the drain region of thetransistor, a fifth impurity region with a fifth volume and a fifthsurface area, the fifth impurity region being of the first conductivitytype; and implanting, into the source region of the transistor, a sixthimpurity region with a sixth volume and a sixth surface area in thesecond surface area of the second impurity region, the sixth impurityregion being of the first conductivity type.

In yet another aspect, the invention is directed to a transistor. Thetransistor includes a semiconductor body with a first conductivity type;a first impurity region with an opposite second conductivity type formedin a surface area of the semiconductor layer; a source including: afirst impurity region of the first conductivity type having a firstvolume and a first surface area, a second impurity region of the firstconductivity type having a second volume and a second surface area,wherein the second volume is embedded in the first volume, and thesecond surface area is surrounded by the first surface area, a thirdimpurity region of the opposite second conductivity type having a thirdvolume and a third surface area, wherein the third volume abuts thesecond volume, and wherein the third surface area is adjacent to thesecond surface area, and a fourth impurity region of the opposite secondconductivity type having a fourth volume and a fourth surface area,wherein the fourth volume overlaps a portion of the third volume and aportion of the first volume, and wherein the fourth surface areaoverlaps a portion of the third surface area, and is at least partiallysurrounded by the first surface area, a drain including a fifth impurityregion of the opposite second conductivity type, the fifth impurityregion having a fifth volume and a fifth surface area; and a gate tocontrol a depletion region between the source and the drain.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a conventional PMOStransistor and NMOS transistor formed on a p-type substrate.

FIG. 2 is a flow diagram illustrating a conventional sub-micron CMOSprocess for manufacturing CMOS transistors.

FIGS. 3A, 3B, and 3C are schematic cross-sectional views of conventionalLDMOS transistors.

FIG. 4 is an exemplary block diagram of a buck switching regulator.

FIGS. 5A-5B are a schematic cross-sectional view of an LDMOS transistorand a three-dimensional view of the surface area of the LDMOS transistorsource and drain regions, respectively.

FIG. 6 is a flow diagram illustrating an exemplary process formanufacturing a semiconductor transistor, including an LDMOS transistor,that is compatible with a sub-micron CMOS process.

FIGS. 7A-7M illustrate the exemplary processes of manufacturing an LDMOStransistor, a PMOS transistor, and an NMOS transistor according to theexemplary processes of FIG. 6.

FIGS. 8A-8C illustrate an exemplary P-body implant step of the exemplaryprocess of FIG. 6.

FIGS. 9A-9B illustrate exemplary shallow drain implant processes.

FIG. 10 is a flow diagram illustrating an alternative process formanufacturing a semiconductor transistor including an LDMOS transistoraccording to a process that is compatible with a sub-micron CMOSprocess.

FIGS. 11A-11H illustrate an exemplary process of manufacturing an LDMOStransistor according to the exemplary process of FIG. 10.

FIG. 12 is a schematic cross-sectional view of an LDMOS transistorhaving a CMOS n-well implant.

FIG. 13 is a schematic cross-sectional view of an LDMOS transistorhaving a CMOS n-well implant as a shallow drain.

FIG. 14 is a schematic cross-sectional view of an LDMOS transistorhaving a DDD implant as a shallow drain.

FIG. 15 is a schematic cross-sectional view of an LDMOS transistorhaving an LDD diffused into source and drains regions of the transistor.

FIG. 16 is a schematic cross-sectional view of an LDMOS transistorhaving a graded shallow drain implant.

FIG. 17 is a schematic cross-sectional view of a p-type LDMOStransistor.

FIG. 18 is a schematic cross-sectional view of a switching circuitincluding a switching circuit having a high-side LDMOS transistor and alow-side LDMOS transistor.

FIG. 19 is a schematic cross-sectional view of a NPN transistor.

FIG. 20 is a flow diagram illustrating an exemplary process formanufacturing the NPN transistor of FIG. 19.

FIG. 21A is a schematic cross-sectional view of an implementation ofhigh-side drive (HSD) circuits with CMOS logic.

FIG. 21B is a circuit diagram of HSD circuits with CMOS logic.

FIG. 22 is a schematic cross-sectional view of a LDMOS transistor withLOCOS on the drain region of the transistor.

FIG. 23 is a flow diagram illustrating an exemplary process forimplanting a P-body of an LDMOS transistor.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 4 is a block diagram of a switching regulator 400 including anLDMOS transistor according to one implementation. Conventional LDMOStransistors typically achieve optimized device performance through acomplex process, such as a BiCMOS process or a BCD process, thatincludes one or more process steps not compatible with a sub-micron CMOSprocess optimized for the mass production of digital sub-micron CMOStransistors. According to one aspect, an LDMOS transistor is providedthat can be fabricated through a process that can be seamlesslyintegrated into a typical sub-micron CMOS process.

Referring to FIG. 4, an exemplary switching regulator 400 is coupled toa first high DC input voltage source 402, such as a battery, by an inputterminal 404. The switching regulator 400 is also coupled to a load 406,such as an integrated circuit, by an output terminal 408. The switchingregulator 400 serves as a DC-to-DC converter between the input terminal404 and the output terminal 408. The switching regulator 400 includes aswitching circuit 410 which serves as a power switch for alternatelycoupling and decoupling the input terminal 404 to an intermediateterminal 412. The switching circuit 410 includes a rectifier, such as aswitch or diode, coupling the intermediate terminal 412 to ground.Specifically, the switching circuit 410 may include a first transistor414 having a source connected to the input terminal 404 and a drainconnected to the intermediate terminal 412 and a second transistor 416having a source connected to ground and a drain connected to theintermediate terminal 412. The first transistor 414 may be aPositive-Channel Metal Oxide Semiconductor (PMOS) transistor, whereasthe second transistor 416 may be an LDMOS transistor.

The intermediate terminal 412 is coupled to the output terminal 408 byan output filter 418. The output filter 418 converts the rectangularwaveform of the intermediate voltage at the intermediate terminal 412into a substantially DC output voltage at the output terminal 408.Specifically, in a buck-converter topology, the output filter 418includes an inductor 420 connected between the intermediate terminal 412and the output terminal 408 and a capacitor 422 connected in parallelwith the load 406. During a PMOS conduction period, the first transistoris closed, and the voltage source 402 supplies energy to the load 406and the inductor 420 through the first transistor 414. On the otherhand, during an LDMOS transistor conduction period, the secondtransistor 416 is closed, and current flows through the secondtransistor 416 as energy is supplied by the inductor 420. The resultingoutput voltage V_(out) is a substantially DC voltage.

The switching regulator also includes a controller 424, a PMOS driver426 and an LDMOS driver 428 for controlling the operation of theswitching circuit 400. The PMOS driver 426 and the LDMOS driver arecoupled to voltage source 430. A first control line 432 connects thePMOS transistor 414 to the PMOS driver 426, and a second control line434 connects the LDMOS transistor 416 to the LDMOS driver 428. The PMOSand NMOS drivers are connected to the controller 424 by control lines436 and 438, respectively. The controller 424 causes the switchingcircuit 400 to alternate between PMOS and LDMOS conduction periods so asto generate an intermediate voltage V_(int) at the intermediate terminal412 that has a rectangular waveform. The controller 424 can also includea feedback circuit (not shown), which measures the output voltage andthe current passing through the output terminal. Although the controller424 is typically a pulse width modulator, the invention is alsoapplicable to other modulation schemes, such as pulse frequencymodulation.

Although the switching regulator discussed above has a buck convertertopology, the invention is also applicable to other voltage regulatortopologies, such as a boost converter or a buck-boost converter, and toRF output amplifiers.

FIG. 5A shows a schematic cross-sectional view of the LDMOS transistor416. The LDMOS transistor 416 can be fabricated on a high voltage n-typewell (HV n-well) 500A implanted in a p-type substrate 502. An HV n-wellimplant is typically a deep implant and is generally more lightly dopedrelative to a CMOS n-well. The HV n-well 500A can have a retrogradedvertical doping profile. The LDMOS transistor 416 generally includes asource region 506, a drain region 508, and a gate 507.

The source region 506 generally includes an p-doped p+ region 515, ann-doped n+ region 517, and a p-doped P-body 522. The drain region 508generally includes an n-doped n+ region 525 and an n-doped shallow drain(N-LD) 527. In some implementations, the source region 506 furtherincludes an n-type lightly doped region 518 (in some contexts, thelightly doped region can be considered to be part of the n+). Thelightly doped region 518 of the LDMOS transistor can be implanted usingsimilar techniques performed with respect to N-LDD regions inconventional CMOS processes, and thus might itself be considered to bean N-LDD region in some contexts. However, this lightly doped regionthat would be the equivalent of an N-LDD in a conventional CMOStransistor is formed only in the source, but not in the drain.Therefore, the lightly doped region 518 will be referred hereinafter asan N-LDS (n-type lightly doped source).

As shown, the N-LDS region 518 overlaps a portion of the n-doped n+region 517, and can extend under the gate oxide 512 further than the n+region 517. In these implementations, the N-LDS can be implanted beforethe formation of the oxide spacer, thus permitting the N-LDS region 518to extend further into the channel than the n+ region 517. The N-LDSregion 518 can be implanted simultaneously and with the same process asthe N-LDD regions in any CMOS transistors on the substrate. Separatemasks can be employed for implanting the N-LDS region 518 and then-doped n+ region 517, thus permitting placing the N-LDS selectively onthe source regions. Alternatively, the N-LDS region 518 and the n-dopedn+ region 517 can be implanted using a same mask to control, forexample, the overlapping region between the N-LDS region 518 and then-doped n+ region 517. In the later implementation, the n-doped n+region 525 in the drain would be implanted using a mask different fromthat used in forming the n-doped n+ region 517 in the source so that noN-LDD is implanted in the drain. Using different masks also can provideflexibility with respect to, for example, the relative dopantconcentration of the n doped n+ region 525 and the n-doped n+ region517.

In some implementations, the N-LDS region 518 can be shallower than then-doped n+ region 517 (i.e., the n-doped n+ region 517 extends deeperinto the substrate 502 than the N-LDS region 518). The boundary of theN-LDS region 518 farther from the gate can be located closer to the gatethan the outer boundary of n-doped n+ region 517, or it can be alignedwith a boundary of the n-doped n+ region 517 and abut the boundary ofthe p-doped p+ region 515 (see, e.g., FIG. 7M)

The HV n-well 500A, the N-LD 527, and the n+ region 525 in the drainregion 508 are volumes containing doped material. Likewise, the n+region 517, the p+ region 515, and the P-body 522 in the source region506 are volumes containing doped material. In some implementations, boththe N-LD 527 and the HV n-well 500A can have a lower concentration ofimpurities than that of the n+ regions 517, 525. Portions at which thesevolumes overlap may have a higher doping concentration than theindividual volumes separately. For example, a portion 524 that containsthe overlapping volumes of the n+ region 525, the N-LD 527, and the HVn-well 500A can have the highest doping concentration among otheroverlapping volume portions. A portion 526 that contains the overlappingvolumes of the N-LD 527 and the HV n-well 500A excluding the n+ region525, can have a lower doping concentration than that of the portion 524.A portion 504 that only includes the HV n-well 500A can have a lowerdoping concentration than that of the portion 524 or 526, because itdoes not include multiple overlapping doped volumes.

With respect to the N-LDS region 518, in some implementations, theportion at which the N-LDS region 518 and the n+ region 517 overlap canhave a higher doping concentration of impurities than the individualvolumes separately. In these implementations, the volume containing theN-LDS region 518 (i.e., N-LDS portion 520) can have a dopingconcentration lower than that of the n+ region portion 516 and/or the p+region portion 514, but a doping concentration higher than that of theP-body 522.

Referring to FIG. 5B, volumes of the p+ region 514, n+ regions 516/525,N-LDS region 520, P-body 522 and N-LD region 527 can each have a surfacearea on the surface 532 of the LDMOS transistor 416. The HV n-well 500Ahas a surface area 534. For example, in the drain region 508, theportion 526 of the N-LD region has a surface area 536 located within thesurface area of the HV n-well 500A. The portion 524 of the n+ region hasa surface area 538 located within the surface area 536 of the portion526 of the N-LD region. In the source region 506, the P-body 522 has asurface area 540 located within the surface area 534. The portion 514 ofthe p+ region and the portion 516 of the n+ region have a surface area544 and 542, respectively, each being located within the surface area540 of the P-body 522.

In implementations where a N-LDS region 518 is diffused into the P-body522, the portion 520 of the N-LDS region 518 also can have a surfacearea 548 located within the surface area 534. The portion of the N-LDSregion 518 overlapping the portion 516 of the n+ region can have asurface area 546 on the LDMOS transistor 416.

FIG. 6 illustrates an exemplary process 600 of fabricating asemiconductor device, including an LDMOS transistor, a PMOS transistorwith floating operation capability (e.g., the source of the transistoris not grounded), and an NMOS transistor with floating operationcapability, that is compatible with a sub-micron CMOS process.

The process 600 begins with forming a substrate (step 602). Thesubstrate can be a p-type substrate or a n-type substrate. Referring tothe example of FIG. 7A, a semiconductor layer including a p-typesubstrate 502 is formed. Next, HV n-wells 500A and 500B for the LDMOStransistor, PMOS transistor with floating operation capability, and NMOStransistor with floating operation capability, as shown in FIG. 7B, areimplanted into the p-type substrate 502 (step 604). In someimplementations, the HV n-wells 500A and 500B can be integrated as asingle well. Alternatively, the HV n-well 500A and the HV n-well 500Bcan be implanted as separate wells. The HV n-well 500A and the HV n-well500B also can be implanted simultaneously or sequentially depending on adesign application.

A CMOS n-well 106, for example, for a PMOS transistor, and a CMOS p-well122, for example, for a NMOS transistor, are implanted into the p-typesubstrate 502, as illustrated in FIG. 7C (step 606). While it isillustrated that the CMOS n-well 106 and the CMOS p-well 122 are formedafter the HV n-wells 500A and 500B, the order can be reversed so thatthe CMOS n-well 106 and the CMOS p-well 122 are formed prior toimplanting the HV n-wells 500A and 500B. In some implementations, the HVn-wells 500A and 500B, and the CMOS n-well 106 can be implantedsimultaneously, for example, by using a single mask. In otherimplementations, each of the HV n-wells 500A and 500B, and the CMOSn-well 106 can be implanted sequentially (and in any order).

Referring to FIG. 7D, a P-body for the NMOS transistor with floatingoperation capability can be implanted (step 608). For example, a P-body700 for the NMOS transistor with floating operation capability can beimplanted into the HV n-well 500B.

After implantation of the P-body 700 for the floating NMOS transistor,the gate oxide for each of the LDMOS transistor, the PMOS transistorwith floating operation capability, the NMOS transistor with floatingoperation capability, and the CMOS transistors, can be formed (step610). In some implementations, each gate oxide can be simultaneously orsequentially formed. For instance, the gate oxide for the LDMOStransistor can be formed at the same time as the gate oxide of the CMOStransistors so that the LDMOS transistor may establish a similarthreshold voltage and gate oxide thickness as those of the CMOStransistors. Alternatively, the gate oxide of the LDMOS transistor canbe formed at a different time or with a different thickness than thegate oxide of the CMOS transistors to flexibly allow the LDMOStransistor to be implemented with a dedicated gate oxide thicknesslarger or smaller than that of the CMOS transistors. In theseimplementations, when the gate oxide of the LDMOS transistor is formedto be thicker than the gate oxide of the CMOS transistors, the LDMOStransistor can allow higher gate drive in applications where a lowervoltage power supply may not be readily available. This flexibilityyields optimization of the LDMOS transistor depending on specificrequirements of a power delivery application, such as efficiency targetsat a particular frequency of operation.

The gate oxide 512 can be formed on the surface 702 of the p-typesubstrate 502 above the HV n-well 500A. Similarly, the gate oxide 706Aof the PMOS transistor (with floating operation capability) and the gateoxide 706B of the NMOS transistor (with floating operation capability)can be formed on the surface of the p-type substrate 502 above the HVn-well 500B. Further, the gate oxide 118 and the gate oxide 134 can beformed on the surface of the p-type substrate 502 above the CMOS n-well106, and on the surface of the p-type substrate 502 above the CMOSp-well 122, respectively.

Next, a polysilicon layer is deposited over the gate oxide (step 612).The polysilicon layer can be used as transistor electrodes forinterconnection purposes. As shown in FIG. 7E, polysilicon layers 510,708A and 708B can be deposited over the gate oxide 512, the gate oxide706A, and the gate oxide 706B, respectively. Also, a polysilicon layer120 and a polysilicon layer 136 are deposited over the gate oxide 118formed above the CMOS n-well 106 and the gate oxide 134 formed above theCMOS p-well 122, respectively.

A self-aligned P-body 522 for the source region of the LDMOS transistoris implanted (step 614). As illustrated in FIG. 7F, the P-body 522 isimplanted into the HV n-well 500A.

In some implementations, the self-aligned P-body 522 is implanted intothe HV n-well 500A in two separate steps to allow for a better controlof vertical depth and amount of lateral side diffusion of the P-body.Referring to FIG. 8A, a first P-body implant 802 of the HV n-well 500Acan be used to limit the vertical depth of the P-body 522. The verticaldepth of the first P-body implant 802 also can control the verticaldoping profile underneath the source region (e.g., 506) of the LDMOStransistor, and therefore can determine the source-to-substratepunch-through breakdown voltage BV_(s) _(—) _(sub) of the LDMOStransistor. The first P-body implant 802 can be a high energy implant.In some implementations, the first P-body implant 802 is implanted usinga large-angle tilt (LAT) implant process. The angle implant tilt can be,for example, about or exceed 7 degrees.

After a first P-body implant 802 is formed, as shown in FIG. 8B, asecond P-body implant 804 is implanted over the first P-body implant802. The second P-body implant 804 can be utilized to control thechannel length of the LDMOS transistor. The second P-body implant 804also can set the surface concentration of the P-body 522 to control thethreshold voltage (V_(t)) of the LDMOS transistor. A subsequent P-bodydrive-in and annealing process that limit the amount of the lateral sidediffusion 806 of the resulting P-body (e.g., for further channel lengthcontrol) is shown in FIG. 8C. In some implementations, the subsequentannealing process is a rapid thermal anneal (RTA) process. A potentialadvantage of the two-step self-aligned P-body fabricated as disclosedherein is that the process does not require a long P-body drive-in athigh-temperatures. Therefore, the process does not significantly affectthe CMOS thermal budget.

Referring to FIG. 7G, a shallow drain (N-LD) 527 is implanted anddiffused into the drain of the LDMOS transistor (step 618). In someimplementations, the shallow drain 527 can be implanted before or afterthe LDMOS gate is formed—i.e., the shallow drain 527 can be non-selfaligned or self aligned with respect to the gate 507 of the LDMOStransistor. The shallow drain 527 can be implanted using the LAT implantor a normal angle tilt implant as discussed above.

In some implementations, as demonstrated in FIG. 9A, the N-LD 527 (andthe n+ region 525) has a spacing 907 from the P-body 522 that can becontrolled by masked gate dimensions. The spacing 907 can be sized suchthat the N-LD 527 extends a predetermined distance d1 from the LDMOSgate. The predetermined distance d1 can be controlled by maskdimensions. In other implementations, as shown in FIG. 9B, the N-LD 527(and the n+ region 525) can extend beyond the right edge of the LDMOSgate by a predetermined distance d2 using a drive-in process.

In some implementations, the N-LD 527 shares the same mask as the HVn-well 500A to reduce the number of mask needed for fabricating theLDMOS transistor. Such an approach is possible if the dopingconcentration of the N-LD 527 is lighter than the P-body 522 so that theextra N-LD implant into the source of the LDMOS transistor does notaffect the channel characteristics.

At step 620, implantation for the N-LDS region 520 is performed,followed by the implantation for the n+ regions at step 624. Once theN-LDS region 520 is formed, as shown in FIG. 7H, the LDMOS transistor isimplanted with an n+ region 525 at the drain and an n+ region 517 at thesource. The n+ regions 710 and 712 are implanted at the drain andsource, respectively, of the NMOS transistor with floating operationcapability. The n+ regions 128 and 130 also are implanted at the sourceand drain regions, respectively, of the CMOS p-well 122.

While it is shown that the N-LDS region 520 is implanted prior toimplantation for the n+ regions, depending on a desired alignment of theN-LDS with respect to the gate, the order can be reversed so that theN-LDS regions 520 is implanted after implantation of the n+ regions. Ineither implementations, the N-LDS region 520 can be shallower than then+ region 517 (i.e., the n+ region 517 extends toward the p-typesubstrate 502 deeper than the N-LDS region 520).

After the N-LDS region 520 and the n+ regions are formed, p+ regions ofthe LDMOS transistor, the PMOS transistor with floating operationcapability, the NMOS transistor with floating operation capability, andthe CMOS transistors, are implanted (step 626). As shown in FIG. 7I, thep+ regions 714A and 714B are implanted at the drain and source,respectively, of the PMOS transistor with floating operation capability.A p+ region 515 is also implanted at the source of the LDMOS transistor.Separate p+ regions 112, 114, are implanted at the source and drain,respectively, of the PMOS transistor. Each of the p+ regions can beformed separately or simultaneously.

FIGS. 7J-7L show the process of step 616 in more detail. Referring toFIG. 7J, after the shallow drain (N-LD) 527 is implanted and diffusedinto the drain of the LDMOS transistor (e.g., step 614), the N-LDSregion 520 is implanted into the source of the LDMOS transistor (step620). The N-LDS region 520 can be implanted to extend under the gateoxide 512 previously formed on the HV n-well. The N-LDS region 520 mayalign with an outer boundary (e.g., the boundary away from the drain) ofthe n+ region 517, and abut the p+ region 515. Alternatively, the N-LDSregion 520 may be implanted with a predetermined distance away from thep+ region 515.

Then, as shown in FIG. 7K, a pair of oxide spacers 530 can be formedadjacent to the gate oxide 512 and the polysilicon 510 (step 622). Afterthe oxide spacers are formed, implantation for the n+ regions isperformed (step 624). The LDMOS transistor can be implanted with an n+region at the drain and another n+ region at the source. The n+ region517 and 525 can be formed over the N-LDS region 520 and the N-LD region527, respectively, as illustrated in FIG. 7L. The n+ regions also can beimplanted at the drain and source of the NMOS transistor with floatingoperation capability, and at the source and drain regions of the CMOSp-well 122. Depending on a design application, the n+ regions can beperformed prior to formation of the oxide spacers.

The p+ region can be formed by a two-step implant in a manner similar tothe n+ region. That is, a N-LDS region can be implanted before formationof the oxide spacer, and a p+ region can be implanted after formation ofthe oxide spacer.

Because the gate may have some finite source/drain overlap, in theseimplementations, the gate (or gate oxide) can be formed first and thenused as a diffusion or implant mask in defining the source and drainregions so as to preclude the source and/or drain from running under thegate oxide. Once the gate is formed, the gate can serve as a mask duringimplantation of the n+ regions and the P-body so that they areself-aligned with respect to the gate. As shown, n+ regions 517 and 525of the LDMOS transistors are implanted and self-aligned with respect tothe corresponding gate oxide.

In some implementations, only one side (e.g., the source) of the LDMOStransistor includes an N-LDS region. For example, the n+ region 525 canbe formed using a one-step process because the drain of the LDMOStransistor does not include a N-LDS region.

In some implementations, steps 602-626 may be performed in the orderlisted, in parallel (e.g., by the same or a different process,substantially or otherwise non-sequentially), or in reverse order toachieve the same result. For example, after forming the p-type substrate502, CMOS n-well 106 and CMOS p-well 122 can be implanted prior toimplanting the HV n-wells 500A and 500B. As another example, the p+regions can be formed prior to implanting the n+ regions, and the N-LDSregion can be formed prior to implanting the N-LD region. As yet anotherexample, the N-LD region 527 can be implanted prior to forming the gateoxide or implanting the self-aligned P-body.

In other implementations, depending on a particular design application,one or more of the steps 602-626, or combinations thereof can bebypassed. In yet other implementations, any of the steps 602-626 may beperformed by two or more processes rather than by a single process.

However, the order discussed above is not limited to that shown. Forexample, the n+ region 517 can be implanted prior to forming the N-LDSregion 520, such that the N-LDS region 520 self-aligns with the gateoxide 512 and overlaps the n+ region 517 after the N-LDS region 520 issubsequently formed.

In some implementations, steps 602-626 may be performed in the orderlisted, in parallel (e.g., by the same or a different process,substantially or otherwise non-sequentially), or in reverse order toachieve the same result. For example, n+/p+ regions can be implantedprior to forming the oxide spacers. As another example, the N-LDS regioncan be implanted prior to implanting the N-LD region. As yet anotherexample, the N-LDS region can be formed prior to or subsequently afterany one of the steps 618, 622, 624 and 626.

In other implementations, depending on a particular design application,one or more of the steps 602-626, or combinations thereof can bebypassed. In yet other implementations, any of the steps 602-626 may beperformed by two or more processes rather than by a single process,performed simultaneously or sequentially.

The processes 600 provides a potential advantage over conventionaltechniques because any channel length variation due to misalignment ofthe P-body 522 and n+ region 518 can be mitigated and compensated by agreater critical dimension (CD) control of the process 600.

Also, PMOS transistors are typically formed on a CMOS n-well. Inapplications where a shift in threshold voltages of CMOS transistors istolerable, a PMOS transistor can be directly implemented in an HVn-well, such as the PMOS transistor with floating operation capabilityin the example of FIG. 7H. Implementing a PMOS transistor directly in anHV n-well has the advantage of allowing the process 600 to skip a CMOSn-well implant and masking step (while maintaining its thermal cycle),thereby potentially lowering the overall process manufacturing cost.

While a single gate has been illustrated, the LDMOS transistor caninclude more than one gate. FIG. 7M illustrates a LDMOS having a dualgate oxide. Referring to FIG. 7M, the LDMOS transistor 750 includes adrain region 734, a source region 736, and a dual gate 720. The dualgate 720 includes a first gate region 722 and a second gate region 724.In one implementation, the first gate region 722 is a controlled gateand the second gate region 724 is non-controlled. A controlled gate is agate that receives a voltage that can activate—i.e., turn on or off—acorresponding transistor. Second gate region 724 can float or becontrolled by the same signal as the first gate region 722. The firstgate region 722 includes a conductive layer 726 and an oxide layer 728,and the second gate region 724 includes a conductive layer 730 and anoxide layer 732. Each of the conductive layers 726, 730 can be a layerof polysilicon. As shown in FIG. 7M, the oxide layer 732 is thicker thanoxide layer 728. The thinner oxide layer provided by the oxide layer 728permits the LDMOS transistor 750 to be controlled by a lower gatevoltage relative to a transistor having a controlled gate with a thickeroxide layer. As shown, the first gate region 722 has a thinner oxidelayer with respect to the second gate region 724 sand the first gateregion 722 abuts the second gate region 724.

Referring again to FIG. 7M, drain region 734 includes an n-doped n+region 740 and an n-doped shallow drain (N-LD) region 738. In someimplementations, the N-LD region 738 is self-aligned with respect to thesecond gate 726. In these implementations, the N-LD region 738 does notcompletely extend underneath the second gate 724. If desired, the N-LDregion 738 can also be non-self aligned.

As a PMOS transistor can be directly implemented in the HV n-well, anNMOS transistor can similarly be implemented within a P-body implant,such as the NMOS transistor with floating operation capability in theexample of FIG. 7H. A sub-micron CMOS process can therefore skip aconventional CMOS P-well implant and masking step (while maintaining itsthermal cycle) to lower the overall process manufacture cost.

FIG. 10 illustrates an alternative process 1000 of fabricating an LDMOStransistor that is compatible with a sub-micron CMOS process.

The process 1000 begins with forming a substrate (step 1002). Thesubstrate can be a p-type substrate or an n-type substrate. Referring tothe example of FIG. 11A, a semiconductor layer consisting of a p-typesubstrate 1102 is formed. An HV n-well for the LDMOS transistor isimplanted into the substrate (step 1004). The implanted well can be anHV (high voltage) n-well, such as HV n-well 1104 (FIG. 11B). A CMOSn-well 106 for a conventional PMOS transistor and a CMOS p-well 122 fora conventional NMOS transistor are implanted into the substrate (step1006) (FIG. 11C). An LDMOS gate oxide and polysilicon is formed for theLDMOS transistor (step 1008) The LDMOS gate oxide and polysilicon isdistinct from the gate oxide and polysilicon of the conventional CMOStransistors (step 1008)—i.e., the gate of the LDMOS transistor is formedseparate from and prior to the formation of the gate of the conventionalCMOS transistors being fabricated at the same time. Referring to theexample of FIG. 11D, the LDMOS gate oxide 1106 is formed on the surface1108 of the substrate on the HV n-well 1104, and a polysilicon layer1110 is deposited over the LDMOS gate oxide.

A self aligned P-body 1112 (with respect to the gate of the LDMOStransistor) for the drain region of the LDMOS transistor is implanted(step 1010). As shown in FIG. 11E, the P-body 1112 is implanted into theHV n-well 1104. The self aligned P-body 1112 can be implanted into theHV n-well in two steps, as discussed above, to allow for a bettercontrol of the vertical depth and the amount of lateral side diffusionof the P-body. The P-body drive-in and annealing process can occur priorto, for example, formation of the gate oxide of the conventional CMOStransistors such that a redesign of the thermal cycle allocated tosub-micron CMOS processes (e.g., process step 206) is not required.

The gate of the conventional CMOS transistors is formed (step 1012).Referring to FIG. 11F, the gate oxide 118 of the conventional PMOStransistor is formed on the surface of the substrate on the CMOS n-well106, and the gate oxide 134 of the conventional NMOS transistor isformed on the surface of the substrate on the CMOS p-well 122. Apolysilicon layer 120 is deposited over the conventional PMOS gate oxide118, and a polysilicon layer 136 is deposited over the conventional NMOSgate oxide 134. A shallow drain is implanted and diffused into the drainof the LDMOS transistor (step 1014). The shallow drain can be non-selfaligned or self aligned. In the example of FIG. 11G, the shallow drainis the n-doped shallow drain N-LD 1114. The N-LD implant can share thesame mask as the HV n-well to reduce the mask count. The n+ regions andp+ regions of the LDMOS transistor are implanted (step 1016). In oneimplementation, during this step, n+ and p+ regions associated with theCMOS transistors are also implanted. As shown in FIG. 11H, a p+ region1116 and an n+ region 1118 are implanted at the source of the LDMOStransistor. An n+ region 1120 is also implanted at the drain of theLDMOS transistor. Further, p+ regions 112, 114, are implanted at thesource and drain, respectively, of the conventional PMOS transistor, andn+ regions 128, 130 are implanted at the source and drain regions,respectively, of the conventional NMOS transistor. As in process 600,formation of the p+ regions and the n+ regions can occur through a 3step process as described above in connection with a sub-micron CMOSprocess.

LDMOS Transistor Performance

The three-way performance tradeoff between the on-resistance R_(dson),the drain-to-substrate breakdown voltage BV_(d) _(—) _(s), and thesource-to-substrate punch-through breakdown voltage BV_(s) _(—) _(sub)of an LDMOS transistor can be improved by using a triple diffusion(N+/N-LD/HV n-well) drain structure that can be fabricated through aprocess compatible with a typical sub-micron CMOS process.

LDMOS transistors can be fabricated on a common HV n-well. A main designrequirement of the common HV n-well is to provide an optimized verticaldoping profile to achieve the highest drain-to-substrate breakdownvoltage BV_(d) _(—) _(sub) and source-to-substrate punch-throughbreakdown voltage BV_(s) _(—) _(sub) as required among all LDMOStransistors being fabricated. For a high voltage LDMOS transistor—e.g.,greater than 30V—the HV n-well is generally deeper and lighter dopedthan a regular (conventional) n-well for the CMOS transistor. Since theHV n-well is implanted at the beginning of the processes 600, 1000, itsformation has no impact on fixed thermal budgets (that have beenoptimized for the mass production of sub-micron CMOS devices) allocatedto sub-micron CMOS processes. An extra drive-in for the HV n-well can beaccommodated if a co-drive-in with a CMOS n-well is not sufficient.Generally, a deep HV n-well with retrograded vertical doping profileoffers the best drain-to-substrate breakdown voltage BV_(d) _(—) _(sub)and source-to-substrate punch-through breakdown voltage BV_(s) _(—)_(sub) performances.

The shallow self aligned diffused drain implant and diffusion (N-LD 512)has a spacing from the P-body implant that is controlled by masked gatedimensions. A main design requirement of the N-LD is to achieve anoptimized lateral doping profile to achieve the best performancetradeoff between the on-resistance R_(dson) and the drain-to-substratebreakdown voltage BV_(d) _(—) _(sub) of the LDMOS transistor. Since theN-LD is a shallow diffusion, it has little impact on the vertical dopingprofile of the LDMOS transistor, and therefore, has little impact on thedrain-to-substrate breakdown voltage BV_(d) _(—) _(sub) andsource-to-substrate breakdown voltage BV_(s) _(—) _(sub) characteristicsof the transistor. The spacing of the N-LD implant from the P-bodyallows for a better control of the drain-to-substrate breakdown voltageBV_(d) _(—) _(sub) by lowering the doping levels at the boundary of theHV n-well/P-body junction. Moreover, such a spacing results in improvedhot carrier injection (HCI) stability of the LDMOS transistor.Generally, a graded lateral doping profile in the drain region of theLDMOS transistor (e.g., as shown in FIGS. 7H and 9) offers a betterperformance tradeoff between the on-resistance R_(dson) and thedrain-to-substrate breakdown voltage BV_(d) _(—) _(sub) than a uniformlateral doping profile. A graded lateral doping profile can be achievedby using a large-angle tilt (LAT) N-LD implant. Furthermore, since adeep drive-in is not required for the N-LD implant, the N-LD can be selfaligned to the gate—i.e., implanted after formation of the LDMOS gate,including gates of the CMOS transistors. Therefore, the addition of theN-LD implant has substantially no impact on fixed thermal budgetsassociated with CMOS process steps (e.g., process step 206).

The above description describes LDMOS transistors having varieddrain-to-substrate breakdown voltage BV_(d) _(—) _(sub) ratings that canbe fabricated in processes compatible with a typical sub-micron CMOSprocess.

The following description describes alternative examples of LDMOStransistors that can be fabricated through processes, such as processes600, 1000, that are compatible with a sub-micron CMOS process.

CMOS N-Well as HV N-Well

An interesting feature of conventional low voltage CMOStransistors—e.g., 3.3V to 5V—fabricated within a sub-micron CMOS processis that the sub-micron CMOS process typically includes implanting a CMOSn-well having a breakdown voltage around 30V. For LDMOS transistorsdesigned for applications of a medium voltage range (e.g., 5V to 25V),these LDMOS transistors can be fabricated on a regular CMOS n-well, thuseliminating a separate HV n-well implant and masking step—i.e., steps604, 1004 of processes 600, 1000, respectively. The remaining steps ofprocesses 600, 1000 can be unaltered.

FIG. 12 shows an example LDMOS transistor 1200 fabricated on a p-typesubstrate 1202 having a CMOS n-well implant 1204 for the LDMOStransistor. The LDMOS transistor 1200 includes a drain region 1206, asource region 1208, and a gate 1210. The drain region 1206 includes ann-doped n+ region 1212 and an n-doped shallow drain (N-LD) 1214. Thesource region 1208 includes an n-doped n+ region 1216, a p-doped p+region 1218, and a p-doped P-body 1220.

CMOS N-Well as N-LD

For LDMOS transistors designed for application in a high voltage range,the HV n-well will typically be much deeper than the regular CMOSn-well. It is therefore possible to substitute the CMOS n-well for theN-LD, thus eliminating the N-LD implant and masking step—i.e., steps618, 1014 of processes 600, 1000, respectively. Therefore, in processes600, 1000 above, a CMOS n-well can be implanted before the gate of theLDMOS transistor is formed, and the CMOS n-well can serve as the shallowdrain and would be non-self aligned with respect to the gate. Theremaining steps of processes 600, 1000 can be unaltered.

FIG. 13 shows an example LDMOS transistor 1300 fabricated on a p-typesubstrate 1302 having a CMOS n-well 1304 as the shallow drain. The LDMOStransistor 1300 has an HV n-well implant 1306 for the transistor. TheLDMOS transistor 1300 includes a drain region 1308, a source region1310, and a gate 1312. The drain region 1308 includes an n-doped n+region 1314 and an n-doped shallow drain (CMOS n-well) 1304. The sourceregion 1310 includes an n-doped n+ region 1316, a p-doped p+ region1318, and a p-doped P-body 1320.

DDD as N-LD

In applications where the sub-micron CMOS process includes fabricationof a DDD (Double Doped Drain) HV-CMOS transistor module, the same DDDimplant can be implemented as the shallow drain of the LDMOS transistorto modulate the resistance of the drain, thus eliminating the N-LDimplant and masking steps 618, 1014 described above. The remaining stepsof processes 600, 1000 can be unaltered. The DDD implant can be selfaligned or non-self aligned with respect to the LDMOS gate. In addition,the DDD implant can have an offset from the P-body implant such that theDDD implant extends a predetermined distance d from the LDMOS gate.

FIG. 14 shows an example LDMOS transistor 1400 fabricated on a p-typesubstrate 1402 having a DDD implant 1404 as the shallow drain. The LDMOStransistor 1400 has a CMOS n-well implant 1406 for the transistor. TheLDMOS transistor 1400 includes a drain region 1408, a source region1410, and a gate 1412. The drain region 1408 includes an n-doped n+region 1414 and an n-doped shallow drain (CMOS n-well) 1404. The sourceregion 1410 includes an n-doped n+ region 1416, a p-doped p+ region1418, and a p-doped P-body 1420.

LDD as N-LD

In a conventional sub-micron CMOS process, a LDD (Lightly Doped Drain)implant and spacer formation step can be introduced to improve NMOStransistor ruggedness against hot electron degradation. In oneimplementation, the LDD implant can be used as the shallow drain for theLDMOS transistor, thus eliminating the N-LD implant and masking steps618, 1014 of processes 600, 1000, respectively. The remaining steps ofprocesses 600, 1000 can be unaltered.

FIG. 15 shows an example of an LDMOS transistor 1500 fabricated on ap-type substrate 1502 having an LDD 1504, 1506 diffused into the sourceregion 1508 and drain region 1510, respectively of the LDMOS transistor.The LDMOS transistor 1500 has an HV n-well implant 1512 for the LDMOStransistor. The LDMOS transistor also includes a gate 1514. The drainregion 1510 further includes an n-doped n+ region 1516. The sourceregion 1508 also includes an n-doped n+ region 1518, a p-doped p+ region1520, and a p-doped P-body 1522.

N-LD Implant Defined by N+ Slit Mask

In one implementation, a graded shallow drain surface implant isachieved by utilizing a slit mask to create multiple standard n+implants spaced apart relative to each other along the surface of theLDMOS transistor in the drain region, thus eliminating the N-LD implantand masking step—i.e., steps 618, 1014 described above. The multiple n+implants in the drain region results in an overall lower doping throughdopant-side diffusion. This implementation is particularly suited forLDMOS transistors with a high breakdown voltage specification. Theremaining steps of processes 600, 1000 can be unaltered.

FIG. 16 illustrates an example of an LDMOS transistor 1600 fabricated ona p-type substrate 1602 having a graded shallow drain surface implant1604. The LDMOS transistor 1600 has an HV n-well implant 1606 for thetransistor. The LDMOS transistor also includes a gate 1608. The drainregion 1610 further includes n-doped n+ regions 1612. The source region1614 includes an n-doped n+ region 1616, a p-doped p+ region 1618, and ap-doped P-body 1620.

P-Type LDMOS Transistor

A p-type high voltage LDMOS transistor can be fabricated. FIG. 17 showsan example a p-type LDMOS transistor 1700 fabricated on a p-typesubstrate 1702. The LDMOS transistor 1700 has an HV n-well implant 1704for the transistor. The LDMOS transistor also includes a gate 1706. Thedrain region 1708 include a p-doped p+ region 1710 and a p-dopedfloating P-well 1712. The floating P-well 1712 can be self-aligned ornon-self-aligned. The source region 1714 includes a p-doped p+ region1716, and an n-doped n+ region 1718. In one implementation, the gateoxide has a uniform thickness (oxide regions 1730 and 1732 have the samethickness). In another implementation, the oxide region 1730 is a thingate oxide and the oxide region 1732 is a thicker gate oxide. In anotherimplementation, the oxide region 1730 is a thin gate oxide and the oxideregion 1732 is a thicker field oxide. While the gate 1706 is shown witha dual gate oxide, if desired, the gate 1706 can be formed as a singleuniform gate.

As with the LDMOS transistor illustrated in FIG. 5A, the p-type LDMOStransistor 1700 is fabricated with a non-self aligned P-body implant1712. More generally, a common feature of the LDMOS transistorsillustrated in FIGS. 12-17 is that the P-body implant is formed prior togate formation of conventional CMOS transistors. This ensures that theLDMOS transistors can be fabricated in a process that is compatible witha sub-micron CMOS process having fixed parameters that have beenoptimized for the mass production of sub-micron CMOS devices.

The availability of complementary p-type LDMOS transistor simplifies thedesign of level shift circuits. The p-type LDMOS transistor, as witheach of the LDMOS transistors described above, can be implemented witheither a thick or thin gate oxide. Referring again to FIG. 17, thep-type LDMOS transistor 1700 is implemented with a thick gate oxide1720. For example, when an LDMOS transistor, such as LDMOS transistor416 (FIG. 5A) is implemented with a high voltage gate—i.e., a gate witha thick gate oxide—a standard high-side p-type transistor (e.g., a PMOStransistor) can be implemented within a switching regulator circuit,thus obviating a need for high-side gate drive considerations. Such anapproach results in a hybrid switching regulator, with a low-side LDMOStransistor and a high-side PMOS transistor that minimizes dynamiccapacitive losses associated with a high-side PMOS pull-up transistor,as illustrated in the switching regulator 400 of FIG. 4. The low-sideLDMOS transistor can have an optimized on-resistance R_(dson) (thin orthick gate oxide). The high-side PMOS transistor can be designed suchthat dynamic capacitive losses typically associated with high-side PMOSpull-up transistors is minimized. In typical DC-DC conversionapplications, in which the conduction duty of the high-side switch isrelatively low, the on-resistance R_(dson) of the high-side transistoris a secondary consideration.

FIG. 18 illustrates a non-hybrid switching regulator 1800 having aswitching circuit 1802 that includes a high-side LDMOS transistor 1804and a low-side LDMOS transistor 1806. The LDMOS transistors 1804, 1806can be fabricated through process 600 or 1000. The switching regulator1800 operates in similar fashion to the switching regulator 400 (FIG.4). However, the switching regulator 1800 includes an LDMOS driver 1808to drive the high-side LDMOS transistor 1804. Generally, the LDMOSdriver 1808 cannot be fabricated using conventional CMOS transistors.However, using through processes 600, 1000, the LDMOS driver 1808 can befabricated using PMOS transistors with floating operation capability andNMOS transistors with floating operation capability. LDMOS driver 428can be fabricated using conventional CMOS transistors, or using PMOStransistors with floating operation capability and NMOS transistors withfloating operation capability. Controller 424 is typically fabricatedusing conventional CMOS transistors.

Other Device Structures

NPN Transistor

Generally, only PNP transistors can be fabricated in a typicalsub-micron CMOS process. However, process 600 can be modified to allowfabrication of an NPN transistor. FIG. 19 shows a cross-sectional viewof an example NPN transistor 1900 that can be fabricated through aprocess compatible with a sub-micron CMOS process.

FIG. 20 illustrates a process 2000 for fabricating an PNP transistor,such as PNP transistor 2000. The process 2000 begins with forming asubstrate (step 2002), such as p-type substrate 1902 (FIG. 19). A wellfor the NPN transistor is implanted into the substrate (step 2004). Theimplanted well can be an HV (high voltage) n-well 1904, as shown in theexample of FIG. 19. A non-self aligned P-body is implanted into thesurface of the transistor (step 2006), which is illustrated as P-body1906 in FIG. 19. The gate oxide and gate polysilicon are formed. The n+regions and p+ regions of the PNP transistor are implanted (step 2008),such as n+ regions 1908 and 1910, and p+ region 1912 (FIG. 19).Alternatively, the P-body 1906 can be self-aligned, and implanted afterformation of the gate oxide and gate polysilicon.

The availability of complementary NPN and PNP transistors enhances highperformance analog circuit design.

CMOS Transistors with Floating Operation Capability

An NMOS transistor with floating operation capability (i.e., the sourceof the NMOS transistor is not grounded) can be implemented throughprocesses 600, 1000, as described above. Such an NMOS transistor,together with a PMOS transistor fabricated in an HV n-well, allows forthe implementation of high-side drive (HSD) circuits (e.g., LDMOS driver1808) with CMOS transistor logic as shown in FIGS. 21A and 21B.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, although some of the LDMOS transistor structures describedabove do not have LOCOS field oxide (FOX) 2202 on the drain region ofthe devices. The processes described above also apply to LDMOStransistor structures with LOCOS on the drain region of the devices suchas LDMOS transistor 2200 shown in FIG. 22. The devices described abovecan be implemented in general half-bridge or full-bridge circuits, andalso in other power electronics systems.

A common feature of the LDMOS transistors described above is that theP-body implant is formed prior to gate oxide formation of conventionalCMOS transistors to ensure that the LDMOS transistors can be fabricatedin a process that is compatible with a sub-micron CMOS process. Asdiscussed above, in one implementation, the P-body can implanted in twosteps using a first high energy implant and a second implant, followedby a RTA process. The first high energy implant can be implanted using aLAT implant. FIG. 23 shows a process 2300 for implanting the P-bodywithout substantially disturbing the CMOS process thermal cycle. Thesecond implant (step 2306), or both the high energy implant (step 2302)and second implant, can occur after gate formation of CMOS transistors(step 2304). The second implant is followed by a RTA process (step2308). The RTA process is implemented with a short duration of time andat temperatures such that thermal cycles allocated to fabricatingsub-micron CMOS transistors are substantially unaffected. As discussedabove, an LDMOS transistor can be fabricated on an n-type substrate. Insuch an implementation, an SOI (silicon-on-insulator) insulation layercan be deposited (or grown) on the n-type substrate. A p-well for theLDMOS transistor and CMOS transistors can then be implanted. The processsteps following formation of the substrate in processes 600, 1000 canthen occur.

Accordingly, other implementations are within the scope of the followingclaims.

1. A method of fabricating a semiconductor device, the methodcomprising: forming an LDMOS transistor on a substrate, the LDMOStransistor including a first drain with a first drain-side n+ region, afirst source with a first source-side n+ region and a first source-sidep+ region, and a first gate between the first drain and the firstsource; and forming an n-type CMOS transistor on the substrate, then-type CMOS transistor including a second drain having a seconddrain-side n+ region, a second source having a second source-side n+region, and a second gate between the second drain and the secondsource; wherein forming the LDMOS transistor and forming the n-type CMOStransistor comprise a first implant step that, without implanting intothe first drain, simultaneously implants a first lightly n-doped regionin the first source to overlap the first source-side n+ region and toextend beneath the first gate, and a second lightly n-doped region inthe second drain to overlap the second drain-side n+ region and toextend beneath the second gate.
 2. The method of claim 1, whereinforming the LDMOS transistor and forming the n-type CMOS transistorcomprise a second implant step that simultaneously implants the firstdrain-side n+ region, the first source-side n+ region, the seconddrain-side n+ region and the second source-side n+ region.
 3. The methodof claim 1, further comprising forming a p-type CMOS transistor on thesubstrate, the p-type CMOS transistor including a third drain having afirst drain-side p+ region, a third source having a second source-sidep+ region, and a third gate between the third drain and the thirdsource.
 4. The method of claim 3, wherein forming the LDMOS transistorand forming the p-type CMOS transistor comprise a second implant stepthat simultaneously implants the first source-side p+ region, the firstdrain-side p+ region and the second source-side p+ region.
 5. The methodof claim 3, further comprising: forming a PMOS transistor on thesubstrate, the PMOS transistor including a fourth drain having a seconddrain-side p+ region, a fourth source having a third source-side p+region, and a fourth gate between the fourth drain and the fourthsource; and forming a NMOS transistor on the substrate, the NMOStransistor including a fifth drain having a third drain-side n+ region,a fifth source having a third source-side n+ region, and a fifth gatebetween the fifth drain and the fifth source.
 6. The method of claim 5,wherein forming the NMOS transistor includes forming the thirddrain-side n+ region and the third source-side n+ region at the sametime as the first drain-side n+ region and the first source-side n+region of the LDMOS transistor and the second drain-side n+ region andthe second source-side n+ region of the n-type CMOS transistor.
 7. Themethod of claim 5, wherein forming the PMOS transistor includes formingthe second drain-side p+ region and the third source-side p+ region atthe same time as the first source-side p+ region of the LDMOS transistorand the first drain-side p+ region and the second source-side p+ regionof the p-type CMOS transistor.
 8. The method of claim 2, furthercomprising forming a gate oxide for each of the LDMOS transistor and then-type CMOS transistor simultaneously to provide a uniform gate oxidethickness between the LDMOS transistor and the n-type CMOS transistor.9. The method of claim 8, further comprising forming oxide spacers,wherein the first implant step is performed prior to forming the oxidespacers and the second implant step is performed after forming the oxidespacers.
 10. The method of claim 1, further comprising forming a gateoxide for each of the LDMOS transistor and the n-type CMOS transistor atdifferent times and such that the LDMOS transistor is formed with a gateoxide thickness different from that of the n-type CMOS transistor.